I had boughten a K&N CAI system for the goood old 4.0 so i bolted it on and about 300 miles later i would get this annoying almost high pitched humming noise. ive had the K&N off for about 1000 miles not and no more hum after i put the stock filter on. i really want to put it back on but i cannot stand the noise. any thoughts on how to clean the IAC? (i thinks that might be the problem)

thanks let me know

 

It could very well have been a leak in the intake somewhere else too. Also consider that most "cold air" intakes like the k&n are actually pulling hotter air into the engine (several people have confirmed by measuring intake air temps with a scan tool). Search as there are plenty of threads about cleaning the IAC valve. Personally, in both cases (97 4.0L and 98 4.0L) I tried to clean the IAC valve on my trucks, it made the noise louder. The noise they produce is usually a fog horn type of noise, I wouldn't really consider it a high-pitched whine, and generally only happens at idle.

 

yes thats exactly it

 

that high pitched whine may just be the air getting sucked into the intake. my rsx did the same thing when it was matted in certain rpm ranges. its almost like a supercharger whine (or so ive been told). if its a consistent whistle type noise at any rpm id think some part of the intake wasnt sealed correctly, or might leak. same theory as blowing over the top of an empty bottle.

 

Sounds like your IAC Valve. I just replaced mine a couple of days ago. It's $60 from AZ.